What Causes an Earthquake?
An earthquake is caused when stress builds up between two large blocks (plates) of the earth’s crust and one of these suddenly slip past the other. The area between the plates are called faults. The earth’s crust is made of several continental and oceanic plates that are constantly rubbing/sliding past each other or pulling apart.
Think of the faults as having jagged edges, similar to how your knuckles on your hand are when you make a fist with both hands. If you gently rub your knuckles together, you can feel them getting locked in place. Then you need to add some extra force to it in order to move again. This is similar to how earthquakes occur and the sudden release of energy is what causes the earth to move and vibrate. Energy is stored up when stuck until enough force is made to move. Then, it will continue to move until it gets stuck again and the process repeats.
What Types of Earthquakes are There?
There are 3 types of faults that cause earthquakes. They can be categorized as Strike-slip, Dip-slip, and Oblique-slip. Each of these work a bit differently so I’ll explain in more detail. More recently, there have been human-induced earthquakes.
This occurs when two plates are sliding horizontally past each other, similar to the knuckle experiment I mentioned earlier. An example is the Great Earthquake that struck San Francisco in 1906 an destroyed 80% of the city.
Dip-slips occur when plates are pushed towards each other and eventually one gives way and either buckles up or buckles down which can create valleys or mountains. There are two types: Normal faults and reverse faults. Normal faults has the footwall moving over the hanging wall whereas in a reverse fault the footwall goes down. Some examples created by these are the Sierra Nevada Valley, Mid-Atlantic Ridge, Rocky Mountains, and the Himalayas.
This is a combination of both strike-slip and dip-slip. Usually any earthquake has some horizontal and vertical movement so categorizing it as oblique-slip means that the measurement of both strike and dip to be significant. An example is the 1999 Hector Mine earthquake that hit California.
Humans are incredible (both in a good way and bad). We are capable of inducing earthquakes by pumping wastewater into deep disposal wells near faults as seen in the incredible amount of earthquakes that strike the Oklahoma region every year. As oil and gas production ramps up in those areas, the number of earthquakes have risen significantly. Evidence of this is that in 2007 Oklahoma experienced only 1 earthquake while last year there were almost 900 3-plus magnitude earthquakes. For the first time, this lead the U.S. Geological Survey to publish an earthquake hazard map covering both natural and “induced” earthquakes.
Now that we know how an earthquake is caused, let’s see how they are located and measured in my next post.
Read more about how to be prepared for an earthquake